You can try with TSVB and the Math aggregation. Then you should be able to calculate the percentage of spam as "100 * sum(spam)/sum(total_volume)".
Basically create 2 sum metrics, one for spam, one for total_volume.
Then create a math aggregation with the equivalent of the formula that I added above, and then keep just this line visible on the chart.
PS: don't forget to keep the bucket size to 1d.